Doc: Update README

[justxuewei]

What this PR does:

• Remove unused images.
• Fix typo.
• Update dubbo-go-cli url from Ftr: Add dubbo-go-cli telnet tool support #818 to https://github.com/dubbogo/tools.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #1294 (14137c8) into 3.0 (968650f) will decrease coverage by 3.31%.
The diff coverage is 55.15%.

❗ Current head 14137c8 differs from pull request most recent head 6b1d9dd. Consider uploading reports for the commit 6b1d9dd to get more accurate results

@@            Coverage Diff             @@
##              3.0    #1294      +/-   ##
==========================================
- Coverage   59.53%   56.22%   -3.32%     
==========================================
  Files         259      273      +14     
  Lines       12737    12779      +42     
==========================================
- Hits         7583     7185     -398     
- Misses       4199     4689     +490     
+ Partials      955      905      -50     

Impacted Files	Coverage Δ
cluster/cluster_impl/available_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/broadcast_cluster_invoker.go	76.47% <ø> (ø)
cluster/cluster_impl/failfast_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/mock_cluster.go	0.00% <0.00%> (ø)
cluster/loadbalance/random.go	100.00% <ø> (ø)
common/extension/auth.go	0.00% <ø> (ø)
common/extension/cluster.go	0.00% <ø> (ø)
common/extension/cluster_interceptor.go	0.00% <0.00%> (ø)
common/extension/config_center.go	0.00% <ø> (ø)
common/extension/config_center_factory.go	0.00% <ø> (ø)
... and 289 more

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[LaurenceLiZhixin]

原来doc文件夹下的照片没用的话就删掉吧，以后统一用 https://github.com/dubbogo/dubbogo.github.io 来放照片和文档。

[justxuewei]

原来doc文件夹下的照片没用的话就删掉吧，以后统一用 https://github.com/dubbogo/dubbogo.github.io 来放照片和文档。

OK, fixed.